I think large numbers of trick or treaters have parents who would rather do the indoor mall circuit-it's dry inside, more people can see the kids in costumes and it's a defined time/night.

It's ok to deal with local kids you know in your neighbourhood. But I could not afford treat for 100+ kids. Maybe 30 tops if being generous. I can get the eye if I dole out one snack size chocolate when 3 might be expected.